This invention relates to resin coated sand for use in the preparation of molds for shell molding operations, and particularly in the improvement of the properties of the molds which prevent their cracking at the time of pouring. In conventional sand-molding operations, a mixture of sand coated with a binder is placed in the mold, and the heat of the processing steps causes reactions to occur between the binder components to improve the pressed strength of the sand and retain the configuration of the part to be cast. After introduction of the molten metal into the cavity, the heat of the metal, during the cooling cycle is transferred to the sandbinder mixture causing the binder to be destroyed to a degree that allows the sand to be removed from the cast metal in an efficient manner.
In the case of iron casting, the stock temperature of shell-mold reaches 800-1000.degree. C. at pouring, and the strength of shell-mold is naturally reduced after casting because almost all the phenolic resin binder is subjected to thermal degradation by the intense heat at pouring. Accordingly it is easy to remove the mold-core from molded articles in the form of sand grains after casting.
Ordinary phenolic resins have been used in these products. They are prepared by mixing heated foundry sand or aggregates with resole type phenolic resins or with novolac type phenolic resins with hexamethylenetetramine.
Said novolac type phenolic resins are generally prepared by reacting 1 mole of phenols with 0.6 to 0.9 moles of aldehyde in the presence of acidic catalysts. The resole type phenolic resins are generally prepared by reacting 1 mole of phenols with 1 to 3 mole of aldehyde in the presence of basic catalysts.
However, there has been an obvious drawback in the conventional phenolic resins to prepare sand molds. The molds often crack at pouring when the phenolic resins are used as the binder. A certain stress occurs on account of an abrupt thermal expansion of coated sand because coated sand is subject to a considerable extent of heat at pouring. The molds are not durable against the stress, and they crack. This is a reason why conventional phenolic resins are disadvantageous as a foundry binder.
A known method to solve the problem to prevent the molds from cracking has been to incorporate cushioning substances into phenolic resins or coated sand obtained therefrom. This method can make the molds flexible as well as free from stress at heating and pouring. Said conventional cushioning substances are Vinsol, bisphenol A, petroleum resins, rosins, etc. While these substances play a role of cushioning effect in the molds to a certain extent, they have drawbacks in that they emit a disagreeable odor at pouring due to thermal decomposition or evaporation. Also, the molds containing such cushioning agents are essentially poor in the shake-out property.
After much investigation to conquer said drawbacks, the inventors hereof have found that the presence of polyethylene glycols with a high molecular weight prevent the molds from cracking at pouring, free from disagreeable odors, and the shake-out property of the molds are not impaired.